If FUSING heights are indicated aloft over theexpected low position, the AMOUNT MUST BEESTIMATED to determine the ACTUAL heightdifference to which the rule will apply. In some casesthe height rises aloft over the expected position of thelow may be quite large, indicating the development of ahigh-latitude ridge aloft, which tends to block theeastward progress of the low. This may result in rapiddeceleration of the low, with falling and/or recurvatureto the north. In such a case, the forecast position of thelow is revised in light of the changing circulation aloft.The above technique works only when lows areexpected to move northeastward out of a heat source,such as the Southern Plains. When a low moves into theSouthern Plains from the west or northwest, there isfrequently no accompanying cooling in the lowtroposphere, since the low is moving toward the heatsource.Some rules for the falling and deepening of surfacelows in relation to upper contours areas follows:l Filling is indicated when a surface low movesl Surface lows tend to fill when the associatedupper-level trough weakens.l Surface lows tend to fill when they move towardvalues of higher thickness lines.l When the associated upper trough deepens, thesurface low also deepens.l Surface lows deepen when they move towardlower thickness values.l Waves develop along fronts when the 700-hPawindflow is parallel to the front, or nearly so.l During periods of southerly flow at 700 hPaalong the east coast of the United States, secondarystorms frequently develop in the vicinity of CapeHatteras.High Tropospheric Divergence inDeveloping LowsIn the case of developing (dynamic) cyclones,horizontal divergence is at a maximum in the 400- tointo or ahead of the major ridge position of the 500-hPa200-hPa stratum, and the air above must sink and warmlevel.adiabatically to maintain equilibrium. See figure 3-8.Figure 3-8.-Vertical circulation over developing low.3-11